Those skilled in the art will be aware that the emissive zone of a laser diode, which zone is constituted by the junction of this diode, is very narrow (of the order of 1 micron only) in the direction perpendicular to the junction and much wider (between 15 and 20 microns) in the junction direction. The coupling of a single optical fibre whose typical diameter is in the order of 100 microns, to a light source of this kind thus depends in a very critical way upon the positioning of the source in relation to the diode, considered in the direction perpendicular to the plane of the junction.
In addition, because of the small thickness of the source, the emitted light beam is highly divergent (around 60.degree.) in the plane perpendicular to the plane of the junction and, in the case of direct coupling between the laser and the fibre aperture, a large proportion of the emitted rays are outside the maximum input angle of incidence so that the coupling efficiency is poor. To improve this efficiency, it has been proposed either to design the aperture of the fibre in such a fashion that it has a hemispherical surface doing duty as a lens or to arrange between the transmission fibre and the diode, a second fibre perpendicular to the first and parallel to the plane of the junction, the function of which second fibre is to act as a cylindrical lens (there is no focussing problem in the plane parallel to the junction, this being the direction in which the beam has low divergence, since the source is much longer than it is wide). In this latter case, the problem arises of the strict positioning of the two fibres in relation to one another.